Astronomical refracting telescope

Consists of two converging lenses:

Needs a ray diagram to explain – revise ray
diagrams.

General points:

oRays from
a distant object (like a star) are always parallel – they will all be
refracted through the focal plane of the “objective lens”.This is the lens that points at the star – the “eye lens” is the
other lens – the one we look through.

oThe two
lenses have “mutual focal points”-
this means that they are put at just the right distance apart so that the focal
point of the objective lens is in the same position as the eye lens

oThe first
point (above) works in reverse – rays coming through the focal plane of the
eye lens will be refracted parallel to a ray through the centre of the lens.

oThe
eyepiece works like a magnifying glass – with the image at infinity

Angular magnification in normal
adjustment

This can be related to the focal lengths of the lenses:

Look at the above diagram again – for angles qo
and q

M=

=

=

small angles (ignore cos) =

Cancel to give:

Do question 4.6 from P. 49 - Ingham

The
eye ring

This
helps us to position our eyes to collect most light coming from the objective
lens.

See
P. 49 – for ray diagram

Explain how refracting telescopes
are designed with an eye ring to gain the best observations

Do
question 4.7 from P. 50 – Ingham.

Attempt
ASSIGNMENT - using the pair of binoculars from P. 50 – Ingham.

List
the limitations of a refracting telescope – P. 50 – Ingham.

Example
question: a telescope has an objective lens with a focal length of 100cm and a
diameter of 5cm.If the eyepiece
has a focal length of 20cm and the telescope is used in normal adjustment,
calculate